The present invention relates generally to pumps, and particularly to power steering pumps for use in vehicle steering systems. Power steering pumps for use in vehicle steering systems are well known and have many different constructions. Normally, such a pump has associated controls for controlling the flow of fluid to a steering system in response to changing pressure demands. The pump also has controls to insure that an excessive amount of fluid flow from the pump is not directed to the steering system.
The present invention specifically relates to a type of power steering pump known as a "cheek plate unloading pump". U.S. Pat. No. 3,822,965 describes and illustrates a pump of this type, which incorporates a movable cheek plate. One side of the cheek plate is presented to the pump displacement mechanism, while the opposite side of the plate faces a fluid pressure chamber. The pressure in the chamber is controlled by a value. The valve is a servo valve that responds to pressure drops in the associated hydraulic system. By controlling the pressure in the chamber, the valve controls the magnitude of forces that act on the cheek plate, and can thereby affect movement of the cheek plate. When the cheek plate moves, fluid is bypassed directly from the outlet of the pump to its inlet, and thus the volume of flow from the pump to an associated hydraulic system is varied. Once a desired and predetermined rate of flow is achieved by a cheek plate unloading pump, the pump maintains the desired flow rate despite variations in pump speed and pressure in an associated hydraulic system.
As noted, the pump of U.S. Pat. No. 3,822,965 uses a servo valve for controlling the flow of fluid from the pump to the associated hydraulic system. The use of a servo valve complicates pump control. The servo valve involves a plurality of parts and is costly. Further, stabilization of the servo valve is necessary. U.S. Pat. No. 4,014,630 discloses a system for stabilizing such a servo valve.